This invention relates to improvements in monolithic catalytic converters, particularly, but not limited to, those used to remove ozone from the conditioned air supplied to aircraft cabins.
Environmental control systems for aircraft supply pressurized and conditioned air to the aircraft cabin. The temperature, pressure, and relative humidity must be controlled to provide for the comfort of flight crew and passengers within the aircraft.
Modern jet aircraft are typically designed for fuel-efficient operation at relatively high altitudes of 25,000 feet or more above sea level. At such altitudes, the ozone content in ambient air is relatively high and thus the air supplied to the aircraft environmental control system can contain a substantial amount of ozone. It can cause lung and eye irritation, headaches, fatigue and/or breathing discomfort.
Catalytic converters have been used to reduce or eliminate undesirable ozone in the air supplied to aircraft cabins. Ceramic monolithic supports have been used which carry catalysts on a washcoat applied to their surfaces, for example, in U.S. Pat. No. 4,405,507. Aluminum honeycomb was treated with NaOH and then solutions of catalytic metals in the ozone converter described in U.S. Pat. No. 4,348,360. In published European patent application 0 388 094 (U.S. Pat. No. 5,080,882) an ozone catalyst is disclosed in which an aluminum surface is oxidized at high temperature to produce an alumina to support a coprecipitated catalyst. Another ozone catalyst was described in published European patent application 0 461 452 (U.S. Pat. No. 5,145,822). In this instance, particulate catalysts were attached by an elastic organic adhesive to a metal foil support.
In co-pending U.S. patent application Ser. No. 07/926,798 an improved catalytic ozone converter of compact size and lightweight construction is disclosed.
The present invention relates to the catalyst employed in such converters, the catalyst being integral with the converter structure and comprising aluminum oxide formed by anodizing the converter structure.
Formation of alumina catalyst by anodization of an aluminum surface was reported by D. Honicke in Applied Catalysis, 5 (1983) 179-198, Elsevier Scientific Publishing Company. Similarly Yamada et al., 8th International Congress on Catalysis, 1984, Vol. IV, p. 835-846, reported an investigation of anodizing aluminum and its use as a catalyst.
The use of an anodized aluminum as a support for automobile catalytic converters was disclosed in Japanese Patent 52 048594. Another method of preparing an anodized surface for use in catalysts is described in European Patent Application 0 281 364. The aluminum is electrolytically etched before anodizing in order to increase the surface area of the alumina film. In this instance, the catalytic metals are also electrolytically deposited as suggested in U.S. Pat. No. 4,472,533.
The monolithic converters of the invention need not be limited to ozone destruction, but also may be employed in many applications where ceramic monoliths have been used. Such ceramic monoliths are commonly coated with a high surface area support which is not required by the present invention.